1. Field Of The Invention
The present invention is drawn to a spray pyrolysis method for producing particles and an apparatus for performing the method. Specifically, the present invention is drawn to a spray pyrolysis method for producing pure metal particles and/or metal oxide particles without using a reducing gas such as hydrogen or carbon monoxide. More specifically, the present invention is drawn to a spray pyrolysis method for producing uniform sized monodispersed particles, particularly microparticles or nanoparticles of a pure metal and/or a metal oxide, from a mixture of a carrier gas and a solution of a metal salt precursor, water and a co-solvent.
2. Description Of Related Art
Metal nanoparticles are of interest for a variety of applications because of their unique chemical, electrical, and optical properties. These applications include catalysis, conducting pastes, templates, and size standards for calibration of optical scattering instruments used by various industries to inspect materials for surface quality. Surface defects such as particulate contaminants and surface roughness, as well as substrate defects, are of great concern for quality control of products such as semiconductor devices, magnetic storage media and flat panel displays. As device dimensions shrink, detection of these defects plays an increasingly important role in increasing product yield. Most studies of defect detection have used optical scattering of dielectric materials such as polystyrene latex (PSL) spheres. Solutions consisting of suspensions of size-monodispersed PSL particles are commercially available, and the PSL particles can easily be deposited onto wafers.
However, it is unlikely that PSL particles are representative of actual industrial process contaminants. Furthermore, the interaction of light between particles and a silicon substrate is much simpler for dielectric materials than for other materials such as metals. A need exists for methods of producing uniform size-monodisperse (xe2x80x9cmonodispersedxe2x80x9d) particles of other materials, particularly metals, which may be deposited onto wafer surfaces for use as optical scattering standards and for evaluating light scattering theories.
Presently in the electronics industry, metal particles have been used in the formation of conductive pastes. These particles, however, have mainly been obtained through chemical precipitation from a solution of metal salt precursor. In order to improve production yield, use of a continuous process such as spray pyrolysis is desirable. However, the reduction of metal oxide, produced from metal precursors, to form pure metal particles is a challenging problem.
The spray process method, which is composed of a carrier gas, an aerosol generator, and a high temperature reactor, has been studied by several research groups. For example, Nagashima et al., Chemical Society of Japan, 1990, 1, 17 and Majumdar et al., Journal of Material Research, 1996, 11, 2861, the entire disclosures of which are incorporated herein by reference, disclose using hydrogen gas as a reducing agent to obtain pure metal particles from various aqueous metal salt systems. However, these methods may be very dangerous in high temperature conditions (above 500xc2x0 C.) because of the explosive property of hydrogen that creates a significant fire hazard. The ignition energy for a hydrogen-air mixture is much lower than for hydrocarbon-air mixtures. Therefore, very low energy sparks such as from a static electric discharge can lead to ignition; furthermore, if the burning gas is even slightly confined, the resulting pressure rise can lead to a detonation.
Several research groups have used hydrogen gas in spray pyrolysis as a reducing gas. Further Xia et al., J. Mater. Res., 2000, 15, 2157, discloses using a co-solvent, ammonium bicarbonate to produce pure Ni particles from an aqueous Ni chloride solution, whereas Nagashima et al., J. Mater. Res., 1990, 12, 2828, discloses using Ni(NO3)2 and NiCl2 aqueous solutions in an H2xe2x80x94N2 atmosphere to produce fine Ni particles. The entire disclosures of these references are incorporated herein by reference.
Copper metal particles have been produced using hydrogen to reduce metal oxide particles formed by spray pyrolysis of copper salt precursors. In these prior art processes, because the concentration of hydrogen required to reduce the metal oxide particles is greater than the flammability limit of hydrogen in the air, a potential safety hazard results.
U.S. Pat. No. 6,316,100 to Kodas et al. (the ""100 patent), discloses a method for producing nickel metal powders. The entire disclosure of the ""100 patent is incorporated herein by reference. The teachings and specific embodiments discloses therein may be used with the present invention.
The ""100 patent is drawn to a method of producing nickel particles that are substantially spherical, have a weight average particle size of not greater than about five gm, a narrow particle size distribution and high crystallinity. The reference is directed to generating an aerosol of droplets including a nickel metal precursor and moving the droplets through a heating zone of 700xc2x0 C.-1400xc2x0 C. to form nickel particles. Embodiments of the reference use a hydrogen, nitrogen and a hydrogen-nitrogen mixture carrier gas. As such, the methods disclosed in the reference may be very dangerous in high temperature conditions for the reasons as stated above with respect to hydrogen. What is needed is a more efficient, less hazardous, method of forming pure nickel particles.
U.S. Pat. No. 5,421,854, also to Kodas et al. (the ""854 patent), discloses a method for manufacturing finely divided particles of palladium, palladium oxide or mixtures thereof The entire disclosure of the ""854 patent is incorporated herein by reference. The disclosed teachings and specific embodiments therein may be used with the present invention.
The ""854 patent discloses a first step of forming an unsaturated solution of a thermally decomposable palladium-containing compound in a thermally volatizable solvent. The reference teaches a following step of forming an aerosol consisting essentially of finely divided droplets of the solution in an air carrier gas. The reference then teaches a third step of heating the aerosol between 300xc2x0 C. and 950xc2x0 C., wherein palladium oxide is formed at temperatures between 300xc2x0 C. and 800xc2x0 C., and pure palladium is formed only at temperatures between 800xc2x0 C. and 950xc2x0 C. What is needed is a more efficient method of forming pure palladium particles.
U.S. Pat. No. 5,861,136 to Glicksman et al. (the ""136 patent), discloses a method for manufacturing fully dense, finely divided, spherical particles of copper I oxide (Cu20) and copper II oxide (CuO) powders. The entire disclosure of the ""136 reference is incorporated herein by reference. The specific teachings and exemplary embodiments therein may be used with the present invention.
The ""136 patent discloses a first step of forming an unsaturated solution of a thermally decomposable copper containing compound in a thermally volatilizable solvent wherein the copper containing compound is used in concentrations not below 0.002 mole/liter or not higher than 90% of saturation, and wherein the particle size of copper I oxide is an approximate function of the cube root of the concentration of the unsaturated solution. The reference teaches a second step of forming an aerosol consisting essentially of finely divided for droplets of the solution in an inert carrier gas. The reference teaches a third step of heating the aerosol, to an operating temperature of at least 1,000xc2x0 C., wherein the copper containing compound is decomposed to form the copper II oxide (CuO), and the copper II oxide is decomposed to formed pure phase copper I oxide (Cu2O) particles. This reference does not disclose a method for producing pure copper particles.
U.S. Pat. No. 6,277,169 to Hampton-Smith et al. (the ""169 patent), discloses a method of providing high quality, micro-size silver-containing particles of a variety of compositions having carefully controlled particle size and size distribution, and an aerosol method for producing the particles. The entire disclosure of the ""169 patent is incorporated herein. The teachings and exemplary embodiments of the reference may be used with the present invention.
The process of the ""169 patent involves processing of a high-quality aerosol including a silver-containing precursor. The aerosol includes droplets of controlled size suspended in and carried by a carrier gas. In a thermal reactor having temperatures ranging from 900xc2x0 C. to 1400xc2x0 C., the liquid of the droplets are vaporized, permitting formation of the desired Pd/Ag particles in an aerosol state. An aerosol at a high droplet loading and at a high metric flow rate is fed to a reactor, where Pd/Ag particles are formed. This reference does not disclose a method for producing pure palladium or pure silver particles.
Another prior art method for generating nanoparticles includes using a solution of a copper salt and water with an inert carrier gas. In particular, in one method, copper acetate in a solution of water is used in a spray pyrolysis system to generate copper nanoparticles. Specifically, the solution of copper nitrate is first dehydrated, as described below in equation (1):
Cu(NO3)2 . 2.5H2Oxe2x86x92Cu(NO3)2+2.5H2O.xe2x80x83xe2x80x83(1)
The dried copper nitrate is then thermally decomposed into copper II oxide in accordance with the following equation (2):
Cu(NO3)2xe2x86x92CuO+2NO2+0.502.xe2x80x83xe2x80x83(2)
The copper II oxide is then reduced by heating the material up to approximately 1000xc2x0 C., which yields copper I oxide and pure copper, in accordance with the following equation (3):
5CuOxe2x86x922Cu2O+Cu+1.502.xe2x80x83xe2x80x83(3)
The problem with the prior art method for decomposition of copper nitrate as discussed above with respect to equations (1)-(3), is that the copper II oxide must be heated up to 1000xc2x0 C., which is not efficient. Furthermore, the prior art method of decomposing copper nitrate yields two moles of copper I oxide for every one mole of pure copper. As such, the method is not preferable when the desired outcome is pure copper metal nanoparticles.
Another prior art method for decomposition of copper includes use of a solution of copper acetate and water in combination with an inert carrier gas. In particular, the copper acetate solution is first dehydrated in accordance with the following equation (4):
Cu(CH3COO)2.H2Oxe2x86x92Cu(CH3COO)2+H2O.xe2x80x83xe2x80x83(4)
The dried copper acetate is then thermally decomposed to copper I oxide in accordance with the following equation (5):
2Cu(CH3COO)2xe2x86x92Cu2O+CO2+others.xe2x80x83xe2x80x83(5)
The copper I oxide is then reduced by heating the material above approximately 600xc2x0 C., thereby yielding pure copper particles, in accordance with the following equation (6):
2Cu2Oxe2x86x924Cu+O2.xe2x80x83xe2x80x83(6)
What is needed is a method for decomposing copper from copper acetate without heating the copper I oxide at temperatures above 600xc2x0 C.
What is also needed is a method for producing metal particles, in particular metal nanoparticles, without the use of hydrogen gas as a reducing agent.
What is additionally needed is a method for making metal particles having a uniform particle size. In particular, what is additionally needed is a method for making pure metal particles having a uniform particle size. More particularly, what is additionally needed is a method for making pure metal nanoparticles having a size distribution that is within a predetermined range.
What is further needed is a method for producing metal particles, in particular metal nanoparticles, more efficiently, i.e., at heating temperatures lower than those of the prior art methods.
It is an object of the present invention to provide metal particles, in particular metal nanoparticles, without the use of hydrogen gas as a reducing agent.
It is an object of the present invention to provide metal particles having a uniform particle size. In particular, it is an object of the present invention to provide pure metal particles having a uniform particle size. More particularly, it is an object of the present invention to provide pure metal microparticles, more preferably nanoparticles, having a size distribution that is within a predetermined range.
It is an object of the present invention to provide metal particles, in particular metal nanoparticles, more efficiently, i.e., at heating temperatures lower than those of the prior art methods.
The present invention provides a method and apparatus for producing metal particles comprising generating aerosol droplets of a solution in an inert carrier gas, and heating the aerosol with a heater thereby forming metal particles, wherein the solution comprises a metal precursor, water and co-solvent reducing agent.
In accordance with one embodiment of the present invention, the metal precursor comprises any one of the group consisting of Fe, Co, Ni, Cu, Zn, Pd, Ag and Au.
In accordance with another embodiment of the present invention, the metal precursor comprises Cu or Ni.
In accordance with another embodiment of the present invention, the co-solvent reducing agent is an organic compound having 1 to 5 carbon atoms.
In accordance with another embodiment of the present invention, the co-solvent is an alcohol. Preferably, the alcohol is present in an amount of about 5% to about 30% by volume of the solution.
In accordance with another embodiment of the present invention, the co-solvent is methanol or ethanol.
In accordance with another embodiment of the present invention, the metal salt precursor is present in an amount of about 0.2 mol/liter to about 2.5 mol/liter, preferably 0.3 mol/liter to about 0.35 mol/liter, of the solution.
In accordance with another embodiment of the present invention, the metal particles are pure metal nanoparticles.
In accordance with another embodiment of the present invention, the metal particles have a diameter within the range of about 50 nm to about 200 nm.
In accordance with another embodiment of the present invention, the water is deionized water.
In accordance with another embodiment of the present invention, the inert carrier gas comprises nitrogen gas.
In accordance with another embodiment of the present invention, the method further comprises passing the heated aerosol through a bipolar charger to obtain metal particles with a Boltzmann charge distribution. Preferably, the method still further comprises passing the heated aerosol through a differential mobility analyzer.
In accordance with another embodiment of the present invention, the step of generating aerosol droplets of a solution further comprises flowing the solution into an atomizer at a flow rate of 20 mL/hr, flowing the inert carrier gas into the atomizer at flow rate of 5 L/min, and atomizing the solution and the inert carrier gas with the atomizer.
In accordance with another embodiment of the present invention, the step of heating comprises passing the aerosol through a two-zone furnace thereby forming metal particles by solvent evaporation and precursor decomposition.
In accordance with another embodiment of the present invention, the step of heating is conducted at a temperature within the range from about 300xc2x0 C. to about 1600xc2x0 C.
In accordance with another embodiment of the present invention, the step of heating is conducted at a temperature within the range from about 450xc2x0 C. to about 800xc2x0 C.
In accordance with another embodiment of the present invention, the inert carrier comprises nitrogen gas.
In accordance with another embodiment of the present invention, the aerosol droplets of solution are dried prior to heating. Preferably, the step of drying comprises passing the aerosol droplets through a screen having a desiccant deposited thereon.
The present invention further comprises a product coated with metal nanoparticles, wherein the metal nanoparticles are produced by the method of generating aerosol droplets of a solution in an inert carrier, drying the aerosol droplets with a drier to obtain a dried aerosol, and heating the dried aerosol with a heater thereby forming metal nanoparticles, and wherein the solution comprises a metal precursor, water and a co-solvent reducing agent.
The present invention further comprises an improvement in a spray pyrolysis method for producing metal nanoparticles from a solution comprising a metal salt precursor and water, wherein the improvement comprises using a co-solvent reducing agent in the solution.
Additional objects, advantages and novel features of the invention are set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.